The present invention relates to a magneto-optical disc system for recording information, sliding a magnetic head along a magneto-optical disc, and also relates to the magneto-optical disc used for such a system.
The basic theory of a magneto-optical recording system is that the temperature of a part of a magnetic thin film is raised to a point exceeding the Curie point or the temperature compensating point, so that coercive force of this portion is extinguished, thereby reversing the direction of magnetization into a direction of recording magnetic field applied from outside. This magneto-optical recording system is being utilized in external recording apparatus of an optical file system or a computer and also recording apparatus for sound and image signals, etc.
A typical magneto-optical disc used for this magneto-optical recording system has a magnetization facilitating axis on a transparent substrate which consists of a polycarbonate etc. in a direction perpendicular to a film side, and forms a recording unit by laminating a magneto-optical recording layer (for example, rare earth-transition metal alloy amorphous thin film) having a large magneto-optical effect, a reflective layer, a dielectric layer, etc., thereby irradiating with laser light from the transparent substrate for reading the signals.
Meanwhile, in general, in said magneto-optical recording system an optical system for irradiating with laser light and a magnetic head impressing a magnetic field are both arranged distantly from a magneto-optical disc. However, when a magnetic field modulation system is employed, sliding the magnetic head along the magneto-optical disc is more useful for low magnetic field recording.
This is based on the following reasons. With the magnetic head impressing a high-speed reversal magnetic field, only a very small magnetic field can be generated because of various restrictions. Therefore, the magnetic head needs to be brought as close to the magnetic recording layer as possible, and it is difficult to control the amount of surfacing of the magnetic head minutely and with high precision.
However, in sliding the magnetic head along the magneto-optical disc, problems such as a focal blank (so-called defocusing) caused by a projection of the magneto-optical disc surface are created.
Namely, when the magnetic recording layer of the magneto-optical disc is covered with a protection film consisting of ultraviolet-ray hardened resin etc., the magnetic head is to slide along the protection film of this magneto-optical disc.
On the other hand, the said protection film is formed by techniques such as spin coat, wherein with the viscosity of ultraviolet-ray hardened resin being high, a bubble tends to remain in the film, causing defects such as a projection.
If the magnetic head slides along the protection film surface having such a projection, the magnetic head will collide with the projection, causing bending (displacement) of the disc and magnetic head jump.
Bending and magnetic head jump will cause a focal blank and shortage of external magnetic field, respectively, thereby disturbing normal recording/reproduction.
In order to solve said problem, all discs having a projection generated with a bubble may be eliminated. However, it is not realistic because of possible deterioration of manufacturing yield.